Printed circuit board design for high speed application

ABSTRACT

A printed circuit board (PCB) is disclosed. The PCB includes a substrate have a top surface and a bottom surface. A first conductive layer is disposed on the top surface of the substrate. The first conductive layer comprises a first signal net and a second signal net. An outermost insulating layer is disposed on the top surface of the substrate to cover the substrate and the first conductive layer. The outmost insulating layer comprises an opening to expose a portion of the second signal net. And, a second conductive layer is disposed on the outermost insulating layer and substantially covering at least a portion of the first signal net. The second conductive layer is filled into the opening to electrically connect to the second signal net which is able to provide one of a ground potential and a power potential.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/451,283, filed Mar. 10, 2011, the entirety of which is incorporatedby reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a printed circuit board and more particularlyto a printed circuit board design with good signal quality.

2. Description of the Related Art

Typically, a printed circuit board (PCB) is a layered insulatingstructure with internal and external traces/planes that allowselectronic devices/components on the printed circuit board to beelectrically connected to each other and to the outside environment.Printed circuit boards are the most commonly used packaging medium forelectronic circuits. Due to the demand for high performance systems,packaging density and propagation speeds have increased, to force thetechnological development of the printed circuit boards to evolve frombeing single-sided to double-sided or multilayered.

Notwithstanding, along with rapid advances in printed circuit boardtechnology, electromagnetic interference (EMI) problems have increasedin severity. When electronic devices/components operate in high speedand are allocated in the PCB with a high device density, noise may begenerated and affect the operation of other electronic devices. In agood printed circuit board design, signal delay, distortion andcrosstalk noise are minimized. Crosstalk is a kind of noise inducedprimarily by the electromagnetic coupling between signal lines anddegrades signal quality. In printed circuit boards, crosstalk occurs bythe electrical coupling between nearby signal traces. As more and morefunctions are integrated on a chip, more circuit traces of the printedcircuit board are demanded, and thus the coupling between nearby signaltraces have become greater, introducing noise and false signals intosystems.

Additionally, referring to FIGS. 8A and 8B, FIG. 8A shows a plan view ofa conventional two-layered printed circuit board 300 which shows routingof power and signal planes. FIG. 8B shows a cross section along line8B-8B′ of FIG. 8A. The conventional printed circuit board 300 has a toplayer on a top surface 102 of a substrate 100, covered by a solder masklayer 126. The top layer comprises power traces 108 a and 108 b and asignal trace 112. The conventional printed circuit board 300 also has abottom layer covered by a solder mask layer 130 comprising a groundplane 140 on a bottom surface 103 of the substrate 100. For descriptiveconvenience, the solder mask layer 126 is not shown in FIG. 8A. Thepower traces 108 a and 108 b are used for providing power potential, andthe signal trace 112 is used for transmitting signal or data. As shownin FIG. 8A, the signal trace 112 is substantially placed along a seconddirection 304 which may form a power transmission barrier between theadjacent power traces 108 a and 108 b which is substantially placedalong a first direction 302, which is not parallel to the seconddirection 304. As shown in FIG. 8B, in order to transmit power potentialbetween the adjacent power traces 108 a and 108 b, a conductive trace108 c is formed on a bottom surface 103 of the substrate 100. Theconductive trace 108 c is respectively electrically connected to thepower traces 108 a and 108 b by via plugs 134 through the substrate 100,and isolated from a split ground plane 140 by a gap 150. As shown inFIG. 8A, the signal trace 112 is across over the gap 150 surrounding theconductive trace 108 c. When signals, especially high-speed signals, aretransmitted along the signal trace 112, however, the current return pathof the high-speed signals not only remains under the signal trace 112,but also transmit along the gap 150 directly under the signal trace 112.The long current return path may generate an undesired magnetic fieldvertical to the directions 302 and 304, and thus increases the couplingcoefficient between adjacent signal traces and exacerbateselectromagnetic interference (EMI) problems. A multi-layered PCB, whichseparates power, signal and ground planes in various layers, may be usedto mitigate the aforementioned problems, but increasing layers of thePCB will also raise the manufacturing cost of the printed circuitedboard.

Accordingly, there is a need to develop printed circuit board designswhich are capable of mitigating the aforementioned problems.

BRIEF SUMMARY OF THE INVENTION

Printed circuit boards are provided. An exemplary embodiment of aprinted circuit board comprises a substrate having a top surface and abottom surface. A first conductive layer is disposed on the top surfaceof the substrate. The first conductive layer comprises a first signalnet and a second signal net. An outermost insulating layer is disposedon the top surface of the substrate to cover the substrate and the firstconductive layer. The outmost insulating layer comprises an opening toexpose a portion of the second signal net. And, a second conductivelayer is disposed on the outermost insulating layer and substantiallycovering at least a portion of the first signal net. The secondconductive layer is filled into the opening to electrically connect tothe second signal net which is able to provide one of a ground potentialand a power potential.

BRIEF DESCRIPTION OF DRAWINGS

The invention can be more fully understood by reading the subsequentdetailed description and examples with references made to theaccompanying drawings, wherein:

FIG. 1 illustrates a cross section of an exemplary embodiment of aone-layered printed circuit board according to the invention;

FIG. 2A illustrates a cross section of an exemplary embodiment of atwo-layered printed circuit board according to the invention;

FIG. 2B illustrates a cross section of another exemplary embodiment of atwo-layered printed circuit board according to the invention;

FIG. 3 illustrates a cross section of an exemplary embodiment of afour-layered printed circuit board according to the invention;

FIG. 4 illustrates a cross section of an exemplary embodiment of asix-layered printed circuit board according to the invention;

FIG. 5A illustrates a simulation diagram showing a relationship betweennear-end crosstalk and frequency for signal traces of two-layeredprinted circuit boards as the embodiment shown in FIG. 2;

FIG. 5B illustrates a simulation diagram showing a relationship betweenreturn loss and frequency for signal traces of two-layered printedcircuit boards as the embodiment shown in FIG. 2;

FIGS. 6A to 6C are an exemplary simulation diagram in time domainshowing an eye-open of one byte of writing data signals transmittedthrough a DDR3 DRAM interface;

FIGS. 7A to 7C are an exemplary simulation diagram in time domainshowing an eye-open of one byte of writing data signals transmittedthrough a DDR3 DRAM interface;

FIG. 8A shows a plan view of a conventional two-layered printed circuitboard; and;

FIG. 8B shows a cross section along line 8B-8B′ of FIG. 8A.

DETAILED DESCRIPTION OF INVENTION

The following description encompasses the fabrication and the purpose ofthe invention. It can be understood that this description is providedfor the purpose of illustrating the fabrication and the use of theinvention and should not be taken in a limited sense. In the drawings ordisclosure, the same or similar elements are represented or labeled bythe same or similar symbols. Moreover, the shapes or thicknesses of theelements shown in the drawings may be magnified for simplicity andconvenience. Additionally, the elements not shown or described in thedrawings or disclosure are common elements which are well known in theart.

In the following various exemplary embodiments, the printed circuitboard may comprise net, traces, strip lines, or micro-strip which aremade of conductive material to electrically connect various electroniccomponents for transmitting data and/or signals in a high speed manner.Such net, traces, strip lines, or micro-strip may be function as ahigh-speed signal bus or interface, such as memory interfaces,high-definition multimedia interface (HDMI), serial advanced technologyattachment (SATA) interface, universal serial bus (USB), Ethernetinterface or low voltage differential signaling (LVDS) interface.Referring to FIG. 1, a cross section of an exemplary embodiment of aone-layered printed circuit board according to one embodiment of theinvention is illustrated. In the embodiment, the printed circuit board200 comprises a substrate 202 made of FR-1, FR-2, FR-4, CEM-1 or CEM-3material. The substrate 202 has a top surface 202 a and a bottom surface202 b. A first conductive layer 203 comprising nets, micro-strips,traces, or strip lines for electrically transmitting signals is formedon the top surface 202 a. The first conductive layer 203 is made ofconductive material comprising copper, aluminum, copper alloy oraluminum alloy. An outermost insulating layer 204 is formed on the topsurface 202 a of the substrate 202 to substantially cover the topsurface 202 a and the first conductive layer 203. In the embodiment, theoutermost insulating layer 204 may comprise a solder mask or othersuitable dielectric materials, e.g. isolation film, isolation tape orpre-preg.

In one embodiment, the first conductive layer 203 comprises a firstsignal net 203 a and a second signal net 203 b and 203 c. In oneembodiment, the first signal net 203 a are electrically connectedbetween electronic components for transmitting signal and/or data. Thesecond signal net 203 b and 203 c are electrically connected to a groundpotential to serve as a ground net. Alternatively, the second signal net203 b and 203 c are electrically connected to a power source having avoltage potential to serve as a power net. The first signal net 203 aand second signal net 203 b and 203 c may be patterned to comprise thefollowing structures, either alone or in any combination, includingtrace, plane, pad, finger, or via hole. The first signal net 203 a andsecond signal net 203 b and 203 c are substantially covered by theoutermost insulating layer 204. In the embodiment of this application,the outermost insulating layer 204 has openings 209 to expose at least aportion of the second signal net 203 b. The exposed portion may comprisea pad, a via hole, a portion of a trace or a portion of a plane of thesecond signal net 203 b.

In particular, a second conductive layer 207 is disposed on theoutermost insulating layer 204 to substantially cover at least a portionof the first conductive layer 203. In the embodiment, the secondconductive layer 207 may be formed of solid metal sheet (such as metalfoil/tape which comprises Al, Cu, Ag, or Au). In another embodiment, thesecond conductive layer 207 can be formed from a conductive epoxy orconductive epoxy adhesive (such as conductive silver, copper, oraluminum epoxy or the like). In another embodiment, the secondconductive layer 207 can be manufactured from a polymer conductivecomposite, conductive polymer, conductive composite polymer, or carbonprinting.

The second conductive layer 207 is electrically connected to the secondsignal net 203 b through openings 209 formed in the outermost insulatinglayer 204 so as to connect to a ground net or a power net. In oneembodiment, the openings 209 may be filled with conductive epoxymaterial or conductive adhesive material so as to form a unity structurewith the second conductive layer 207. In another embodiment, aconductive device (not shown), e.g. screw, pin, jumper etc., is placedthrough the opening 209 to electrically connect the second conductivelayer 207 and second signal net 203 b. In addition, in yet anotherembodiment of the application, the second conductive layer 207electrically connect to second signal net 203 b through not only theconductive device but also the conductive epoxy material filled into theopenings 209. As a result, the second conductive layer 207 can serve asa reference plane to suppress crosstalk noise between first signal netsnearby.

Crosstalk noise which is raised by adjacent first signal net 203 a canbe virtually suppressed by the second conductive layer 207 (i.e., thereference plane) in the micro-strip geometry. Note that in order toeffectively suppress or eliminate the crosstalk noise between theadjacent first signal net 203 a, in one embodiment, a spacing H1 betweenthe second conductive layer 207 and the first signal net 203 a on thesubstrate 202 is less than 1 mm, preferable in a range of 50 μm to 350μm, such that the first signal net 203 a are substantially close to thesecond conductive layer 207 in a vertical direction with respect to thetop surface 202 a of the substrate 202. Additionally, an optional firstinsulating cap layer 208 may be disposed on the outermost insulatinglayer 204 to cover the second conductive layer 207. The first insulatingcap layer 208 may comprise the same or similar material as that of theoutermost insulating layer 204 to protect the underlying secondconductive layer 207 from mechanical or chemical damage.

Referring to FIGS. 2A and 2B, which illustrate cross sections of variousexemplary embodiments of a two-layered printed circuit board. Elementsin FIGS. 2A and 2B that are similar to those in FIG. 1 are labeled withthe same reference numbers as in FIG. 1 and are not described again forbrevity. In FIG. 2A, the two-layered printed circuit board 400 acomprises an outermost insulating layer 210 disposed on the bottomsurface 202 b of the substrate 202. The outermost insulating layer 210may comprise the same or similar material as that of the outermostinsulating layer 204. Moreover, a third conductive layer 309 is disposedon the bottom surface 202 b of the substrate 202 and is embedded in theoutermost insulating layer 210. Typically, the third conductive layer309 may comprise same or similar material as that of the firstconductive layer 203 and serves as a ground or power net.

In one embodiment, the second conductive layer 207 may be electricallyconnected to the third conductive layer 309 through openings 209, secondsignal net 203 b, and via holes 201. The openings 209 are filled withconductive epoxy material or conductive adhesive material so as to forma unity structure with the second conductive layer 207. As a result,since the second conductive layers 207 and the third conductive layer309 can serve as reference planes, crosstalk noise between the adjacentfirst signal nets 203 a of the printed circuit board 400 a can bevirtually suppressed by the second conductive layer 207 and the thirdconductive layer 309 in the micro-strip geometry.

In FIG. 2B, the two-layered printed circuit board 400 b comprisesoutermost insulating layers 204 and 210 disposed on the top and bottomsurfaces 202 a and 202 b of the substrate 202, respectively. A firstconductive layer 303 comprises a plurality of non-ground net 303 a (suchas signal net and/or power net) and at least one ground net 303 b whichare embedded in the outermost insulating layer 204. The plurality ofnon-ground net 303 a are substantially coplanar with the ground net 303b. Moreover, the plurality of non-ground net 303 a and the ground net303 b are formed of the same conductive material, such as copper,aluminum, copper alloy or aluminum alloy.

A third conductive layer 309 comprises at least one non-ground net 309 a(such as signal net or power net) and a ground net 309 b. The non-groundnet 309 a and the ground net 309 b are disposed on the bottom surface202 b of the substrate 202, and are embedded in the outermost insulatinglayer 210, in which the non-ground net 309 a is placed adjacent to theground net 309 b. Moreover, that the non-ground net 309 a is isolatedfrom the ground net 309 b by gap 311. Additionally, the ground net 309 bis electrically connected to the ground net 303 b through a via hole 201b formed in the substrate 202. The non-ground net 309 a is electricallyconnected to one of the plurality of non-ground net 303 a by a via hole201 a formed in the substrate 202. The ground net 309 b may be patternedto comprise the following structures, either alone or in anycombination, including trace, plane, pad, finger, or via hole.

A long current return path due to the split ground net 309 b maygenerate an undesired magnetic field, and thus increases the couplingcoefficient between adjacent signal nets and exacerbates electromagneticinterference (EMI) problems (S. H. Hall, G. W. Hall, and J. A. McCall,High-Speed Digital System Design. New York, N.Y.: John Wiley & Sons,2000, ch. 6.). Accordingly, in the embodiment, a fourth conductive layer215 serving as a reference plane is disposed on the outermost insulatinglayer 210 corresponding to a region/area W of the substrate 202, suchthat the fourth conductive layer 215 substantially covers the pluralityof non-ground net 303 a. The fourth conductive layer 215 may be formedof solid metal sheet (such as metal foil/tape which comprises Al, Cu,Ag, or Au). In another embodiment, the fourth conductive layer 215 canbe formed from a conductive epoxy or conductive epoxy adhesive (such asconductive silver, copper, or aluminum epoxy or the like). In anotherembodiment, the fourth conductive layer 215 can be manufactured from apolymer conductive composite, conductive polymer, conductive compositepolymer, or carbon printing.

The fourth conductive layer 215 is electrically connected to the groundnet 309 b through openings 221 formed in the outermost insulating layer210 so as to connect to a ground potential. In one embodiment, theopenings 221 may be filled with conductive epoxy material or conductiveadhesive material so as to form a unity structure with the fourthconductive layer 215. In another embodiment, a conductive device (notshown), e.g. screw, pin, jumper etc., is placed through the opening 211to electrically connect the fourth conductive layer 215 and ground net309 b. In addition, in yet another embodiment of the application, thefourth conductive layer 215 may electrically connect to ground net 309 bthrough not only the conductive device but also the conductive epoxymaterial filled into the openings 221.

Coupling coefficient between the adjacent non-ground net 303 a (such assignal net) can be reduced and the EMI can be mitigated by the fourthconductive layer 215 (i.e., the reference plane) in the similarmicro-strip geometry. Note that in order to effectively suppress thecrosstalk noise (i.e., the coupling coefficient) and mitigate the EMI, aspacing H2 between the fourth conductive layer 215 and the thirdconductive layer 309 is less than 1 mm, preferable ranged between 50 μmto 350 μm, such that the ground net 309 b and the non-ground net 309 aare substantially close to the fourth conductive layer 215 in a verticaldirection with respect to the bottom surface 202 b of the substrate 202.Additionally, an optional second insulating cap layer 216 may bedisposed on the outermost insulating layer 210 to cover the fourthconductive layer 215. The second insulating cap layer 216 may comprisethe same or similar material as that of the outermost insulating layer210 to protect the fourth conductive layer 215 from mechanical orchemical damage.

Referring to FIG. 3, which illustrates a cross section of an exemplaryembodiment of a four-layered printed circuit board according to theinvention. Elements in FIG. 3 that are similar to those in FIGS. 1, 2Aand 2B are labeled with the same reference numbers as in FIGS. 1, 2A and2B and are not described again for brevity. In the embodiment, theprinted circuit board 500 comprises a substrate 202. Outermostinsulating layer 204 and 210 are disposed on the top and bottom surfaces202 a and 202 b of the substrate 202, respectively, as the embodimentshown in FIG. 2A or 2B.

Unlike the embodiment shown in FIG. 2A or 2B, third signal net 212 a andfourth signal net 212 b and 212 c are embedded in the outermostinsulating layer 210. Moreover, the third signal net 212 a and thefourth signal net 212 b and 212 c are formed of the same conductivelayer, such as copper, aluminum, copper alloy or aluminum alloy. Thefourth signal net 212 b and 212 c may be configured as traces or planes,and partially with pads, fingers, or via holes. In the embodiment of theapplication, the outermost insulating layer 210 has openings 410 toexpose at least one pad, via hole, a portion of a trace or plane of thefourth signal net 212 b.

A first reference plane 403 (such as a ground or power net) is embeddedin the substrate 202 and is electrically connected to the second signalnet 203 b by via holes 401 formed in the substrate 202. A secondreference plane 404 (such as a power or ground net) is embedded in thesubstrate 202 and is electrically connected to the fourth signal net 212b by via holes 402 formed in the substrate 202.

In one embodiment, the second signal net 203 b may be electricallyconnected to ground potential (i.e., the first reference plane 403serves as a ground net), while the second signal net 203 c areelectrically connected to a power source having a voltage potential. Thefourth signal net 212 b may be electrically connected to a power source(i.e., the second reference plane 404 serves as a power net), while thefourth signal net 212 c are electrically connected to the groundpotential. Alternatively, the second signal net 203 b may beelectrically connected to a power source (i.e., the first referenceplane 403 serves as a power net), while the second signal net 203 c areelectrically connected to the ground potential. The fourth signal net212 b may be electrically connected to the ground potential (i.e., thesecond reference plane 404 serves as a ground net), while the fourthsignal net 212 c are electrically connected to a power source.

In particular, the second conductive layers 207 and the fourthconductive layer 215 are disposed on the outermost insulating layers 204and 210 corresponding to the region/area W, such that the secondconductive layer 207 substantially covers the first signal net 203 a,and second signal net 203 b and 203 c, while the fourth conductive layer215 substantially underlies the third signal net 212 a, and the fourthsignal net 212 b and 212 c. In another embodiment, the second signal net203 c and the fourth signal net 212 c may be outside the region/area W.The fourth conductive layer 215 is electrically connected to the fourthsignal net 212 b through openings 410 formed in the outermost insulatinglayer 210. In one embodiment, the fourth conductive layer 215 iselectrically connected to the ground potential when the fourth signalnet 212 b and the second reference plane 404 serve as a ground net,while the second conductive layer 207 is electrically connected to apower source when the second signal net 203 b and the first referenceplane 403 serve as a power net. In this case, the fourth conductivelayer 215 may be electrically connected to ground through the openings410 by a screw, pin, jumper, conductive adhesive or conductive epoxy. Inanother embodiment, the fourth conductive layer 215 is electricallyconnected to the power source when the fourth signal net 212 b and thesecond reference plane 404 serve as a power net, while the secondconductive layer 207 is electrically connected to the ground potentialwhen the second signal net 203 b and the first reference plane 403 serveas a ground net. As a result, the first signal net 203 a and the thirdsignal net 212 a have a strip-line configuration.

Crosstalk noise between the first signal net 203 a and that of betweenthe third signal net 212 a can be virtually suppressed by the respectivemicro-strip geometry. Also, the spacing H1 and H2 are less than 1 mm,preferable in a range of 50 μm to 350 μm, such that the first signal net203 a and the second signal net 203 b and 203 c are substantially closeto the second conductive layers 207 in a vertical direction with respectto the top surface 202 a of the substrate 202, while the third signalnet 212 a and the fourth signal net 212 b and 212 c are substantiallyclose to the fourth conductive layers 215 in a vertical direction withrespect to the bottom surface 202 b of the substrate 202 . Additionally,an optional second insulating cap layer 216 may be disposed on theoutermost insulating layer 210 and covers the fourth conductive layer215 to protect the fourth conductive layer 215 from mechanical orchemical damage.

Referring to FIG. 4, which illustrates a cross section of an exemplaryembodiment of a six-layered printed circuit board according to theinvention. Elements in FIG. 4 that are similar to those in FIG. 3 arelabeled with the same reference numbers as in FIG. 3 and are notdescribed again for brevity. Comparing to the embodiment of FIG. 3, thesix-layered printed circuit board 600 further comprise a plurality offifth signal net 501 and a plurality of sixth signal net 503 embedded inthe substrate 202, in which the plurality of the fifth signal net 501are disposed between the first reference plane 403 and the outermostinsulating layer 204, and the plurality of the sixth signal net 503 aredisposed between the second reference plane 404 and the outermostinsulating layer 210.

Referring to FIGS. 5A and 5B, FIG. 5A is a simulation diagram showing arelationship between near-end crosstalk (dB) and frequency (GHz) forsignal traces of two-layered printed circuit boards as the embodimentshown in FIG. 2, and FIG. 5B is a simulation diagram showing arelationship between return loss and frequency for signal net oftwo-layered printed circuit board for signal traces of two-layeredprinted circuit boards as the embodiment shown in FIG. 2. In FIGS. 5Aand 5B, the curve A represents signal net of a conventional two-layeredprinted circuited board without any additional conductive layer disposedon the outermost insulating layer thereon. The curves B and C representfirst signal net 203 a of a two-layered printed circuited board 400 a or400 b with a second conductive layer 207 having a thickness of 100 μmand a spacing H1 of 80 and 110 μm, respectively (as shown in FIG. 2A),and with a fourth conductive layer 215 on an outermost insulating layer210 having a thickness of 100 μm and a spacing H2 of 80 and 110 μm,respectively (as shown in FIG. 2B). As shown in FIGS. 5A and 5B, thenear-end crosstalk and return loss of the signal net during highfrequency operations can be reduced by the additional conductive layers207 and 215.

According to the aforementioned embodiments, since there is anadditional conductive layer, which is connected to a ground potential ora power source, disposed on the outermost insulating layer of theprinted circuit board and close to the high-speed signal net embeddedtherein, the crosstalk noise and EMI can be effectively suppressed.Moreover, the formation of such an additional conductive layer (servingas a reference plane) is easy and has a lower fabrication cost whencompared with using multi-layered PCBs (such as a 4-layer PCB) withseparated power, signal and ground net in various layers. Accordingly,an economic benefit can be obtained by using of a less-layer PCB withthe mentioned additional conductive layer for high-speed applications.

Please refer to FIGS. 6A to 6C and Table 1. FIGS. 6A to 6C are anexemplary simulation diagram in time domain showing an eye-open of onebyte of writing data signals transmitted through a DDR3 DRAM interfaceat 1350 Mb/s with an on-die termination of 120 ohms at the DRAM side.Table 1 shows the measurement result of the eye-open shown in FIGS. 6Ato 6C. As shown in FIGS. 6A to 6C and Table 1, the larger eye open makesthe better voltage and timing margins.

TABLE 1 Type Spacing Conventional Spacing (H1, H2) = Item design (H1,H2) = 80 μm 110 μm V_(IH(dc)) margin (mV) 160 250 270 V_(IL(dc)) margin(mV) 220 250 286 Overshoot (V) 1.61 1.56 1.55 Undershoot (V) −0.11 −0.13−0.12 Skew (ps, V_(ref) to V_(ref)) 324 244 221 H. Eye-open (ps) 284 394406

Please refer to FIGS. 7A to 7C and Table 2. FIGS. 7A to 7C are anexemplary simulation diagram in time domain showing an eye-open of onebyte of writing data signals transmitted through a DDR3 DRAM interfaceat 1350 Mb/s without on-die termination at the DRAM side. Table 2 showsthe measurement result of the eye-open shown in FIGS. 7A to 7C. As shownin FIGS. 7A to 7C and Table 2, the larger eye open makes the bettervoltage and timing margins.

TABLE 2 Type Spacing Conventional Spacing (H1, H2) = Item design (H1,H2) = 80 μm 110 μm VIH(dc) margin (mV) 107 280 320 VIL(dc) margin (mV)157 279 315 Overshoot (V) 2.15 1.92 1.94 Undershoot (V) −0.63 −0.50−0.52 Skew (ps, Vref to Vref) 529 330 322 H. Eye-open (ps) 153 332 351

While the invention has been described by way of example and in terms ofpreferred embodiment, it is to be understood that the invention is notlimited thereto. To the contrary, it is intended to cover variousmodifications and similar arrangements (as would be apparent to thoseskilled in the art). Therefore, the scope of the appended claims shouldbe accorded the broadest interpretation so as to encompass all suchmodifications and similar arrangements.

1. A printed circuit board, comprising: a substrate having a top surfaceand a bottom surface; a first conductive layer disposed on the topsurface of the substrate, wherein the first conductive layer comprises afirst signal net and a second signal net; an outermost insulating layerdisposed on the top surface of the substrate to cover the substrate andthe first conductive layer, wherein the outmost insulating layercomprises an opening to expose a portion of the second signal net; and asecond conductive layer disposed on the outermost insulating layer andsubstantially covering at least a portion of the first signal net,wherein the second conductive layer is filled into the opening toelectrically connect to the second signal net which is able to provideone of a ground potential and a power potential.
 2. The printed circuitboard of claim 1, further comprising a third conductive layer disposedon the bottom surface of the substrate, wherein the third conductivelayer comprises a third signal net and a fourth signal net, the fourthsignal net is electrically connected to the second signal net through afirst via hole formed in the substrate.
 3. The printed circuit board ofclaim 2, wherein the third signal net is electrically connected to thefirst signal net through a second via hole formed in the substrate. 4.The printed circuit board of claim 1, further comprising a referenceplane embedded in substrate wherein the reference plane is able toprovide one of the ground potential and the power potential.
 5. Theprinted circuit board of claim 4, wherein the reference plane iselectrically connected to the second signal net by a via hole formed inthe substrate.
 6. The printed circuit board of claim 1, wherein thesecond conductive layer comprises metal sheet, conductive epoxy polymerconductive composite, or carbon printing.
 7. The printed circuit boardof claim 1, wherein the second conductive layer is electricallyconnected to the second signal net by one of a screw, pin, solder, andconductive adhesive.
 8. The printed circuit board of claim 1, whereinthe second signal net comprises trace, plane, pad, finger, or via hole.9. The printed circuit board of claim 1, further comprising aninsulating cap layer covering the second conductive layer.
 10. Theprinted circuit board of claim 1, wherein the thickness of outermostinsulating layer is less than 1 mm.
 11. The printed circuit board ofclaim 1, wherein the thickness of second conductive layer is less thanor equal to 100 μm.